1. The Field of the Invention
The present invention relates to an apparatus and method of testing float equipment used in the construction of wells for the hydrocarbon exploration and production industry, and in particular to an apparatus and method for testing the pressure integrity of float equipment, including but not limited to float shoes and float collars used in cementing operations.
2. The Relevant Technology
In the construction of oil and gas wells, drillable non-return back-pressure valves are used to prevent liquid cement flowing back inside wellbore casing and to prevent fluids from flowing into a range of tubulars during run-in. The valves are usually one way valves located towards the end of the tubular or casing string so fluid can be pumped out of the pipe but cannot flow back inside. The drillable non-return back-pressure valves are placed inside short purpose built sections of pipe that match the characteristics of the rest of the tubing string, and are usually closed with the assistance of a spring. Once closed the pressure exerted on them by the column of fluid on the outside of the tubular keeps them closed.
The deepest non-return valve is inside a tubular usually referred to as a float shoe. The float shoe defines the bottom of the tubular and often has a rounded nose profile at its end to act as a guide to help the deployment of the pipe. When running casing, it is also common to have a least a second separate non-return valve above the float shoe, in an upper device referred to as a float collar. The float collar often has a landing surface for a drillable wiper displacement plug to prevent further pumping and displacement of the cement slurry. The float collar usually has an upper and lower threaded connection so that it can be inserted between sections of casing at a pre-determined position above the float shoe.
Float equipment is used in one form or another in various different sizes in nearly all oil or gas wells that are drilled, and in particular when running and cementing casing. It is common for three to five different sets of float sizes to be used in one well (corresponding to different casing diameters), although some wells may use more than eight sets.
It is important to the success of the cementing operation that the non-return valves function correctly. They must open when pumping commences relatively easily and allow significant volumes of drilling mud and liquid cement to pass through them. They must close at the end of the cementing operation and they must withstand significant back pressure. They must often also withstand elevated temperatures found in the rock at significant drilled depths. They must be compatible with various types of chemicals, oils and water found in drilling muds as well as with the liquid cement slurry. They must also tolerate certain amounts of debris, scale, metal filings that can be found in oil and gas wells as a result of drilling operations.
The float collar and its integral drillable landing surface must also be capable of withstanding an applied load as it is common practice to apply a positive pressure on top of a wiper plug to check the pressure integrity of the casing that has been deployed.
The non-return valves are also important for the safety of the well construction process, as they prevent unwanted fluids or gases from entering the casing during installation so that they do not vent out of the top of the casing. If the valves failed to close properly, there would be a risk of uncontrolled volumes of flammable liquid hydrocarbons or gas entering the casing and venting to surface.
To enable drilling to continue beyond the casing depth, the float shoe and float collar must typically remain drillable with commonly used rock drilling tools (although occasionally float equipment may be manufactured from non-drillable materials when it is known that it is the last set of equipment to be deployed in the well and it will not be drilled through).
Examples of traditional float shoes are described in U.S. Pat. No. 1,937,188, U.S. Pat. No. 2,008,818, and U.S. Pat. No. 2,117,318. U.S. Pat. No. 1,937,188 relates to a float shoe which comprises an aluminium body to allow it to be drilled out after the casing has been set. An anchor is provided in the guide shoe to reduce the tendency of the shoe to rotate with the drilling tool when being drilled. The float shoe is joined to a pipe connection by pouring a molten material between respective parts to form a permanent connection, and voids in the show are filled with cement.
A wide range of float equipment is available on the market, and modern designs include advanced functionality and additional features. Modern float equipment often includes non-return valves formed from plastic material, such as a Nylon-based plastic or phenolic resin, aluminium, rubbers, and various combinations of these materials. The hull of the float shoe or float collar is usually a steel material that matches the dimensions and specifications of the casing to which it becomes attached. The steel threaded hull is usually grooved on its inner bore and the valves—which are drillable—are positioned inside. Liquid cement is poured into the hull around the outside of the valve to secure it in the float equipment in a way that ensures that the inlet and outlet of the valve are free of cement. A tubular port extends through the shoe to allow for fluid transfer through the valve. The valves may alternatively be secured and sealed inside the steel hull by a threaded or pinned fixture.
The importance of float equipment to the successful and safe completion of oil and gas wells has led the industry to develop acceptance guidelines for the manufacture and functionality of float equipment. The guidelines include a number of performance acceptance tests that qualify the float equipment as suitable for use in oil and gas wells. These acceptance tests focus mainly on the durability and integrity of the valve. The tests include (a) pumping different fluids at various rates for a range of durations to simulate casing cementing operations and (b) applying heat to the valves. Subsequently pressure tests are applied including a low pressure tests and high pressure tests that simulate the conditions the valves have to endure during and after cementing. The oil and gas industry generally only uses float equipment designs with valves that have passed these qualifications tests, although some unqualified valves may still be used from time to time.
Once a valve type has been qualified this design is manufactured and used in float equipment widely. However, float equipment fails from time to time when it is in the well. These failures may occur for a number of different reasons. Poor quality assurance routines during the manufacture and assembly of the float equipment may cause the equipment to fail, or failure may result from the procedures of the manufacturer not being followed correctly. The float equipment may be subjected to a significant shock load during transportation from the manufacturing plant to the country of use without being reported. The float equipment may also become damaged during the threading procedure after it has been assembled. The drillable non-return valves may fail due to use of faulty materials in the construction of the valve, or the cement slurry used to secure the valve in place may not have been mixed correctly. There are a number of ways in which the valve may fail when under load in the oil or gas well which are not obvious to the human eye when visually inspecting or manually functioning the item.
Some rudimentary checks usually take place at the time of installation into the well. These checks are performed very near the surface of the well before the casing is run any significant distance, and involve lowering the end of the casing string with the float shoe on into the well fluid and seeing if the casing fills with drilling mud. This would indicate that the valve is leaking, so if the casing does not fill above the valve, this check is deemed successful. A similar check involves filling the inside of the casing above the float shoe with drilling mud and then lifting up the shoe out of the top of the well to see if it drains. This would indicate that the valve is opening, so if the mud exits this test is deemed successful. This process is then usually repeated with the float collar is attached to the casing, prior to the rest of the casing being run.
While these checks may identify some serious malfunctions of the valve, they do not properly replicate the conditions to which the equipment will be exposed when in the wellbore. They do not load the valve or the mechanism of its attachment to any significant value and are unlikely to indicate any weaknesses in the equipment. It is only once the equipment has been run to the bottom of the well into the hole drilled in the rock and the cement job performed that it will be loaded towards its design rating.
Unfortunately, a failure that only manifests itself due to loading at the end of the cementing operation is not easily dealt with. The equipment is submerged a significant distance in the well and surrounded by liquid cement that is going to harden in a number of hours. The cement will thicken significantly before it hardens and will prevent further pumping or withdrawal of the casing string. Under these circumstances the only usual remedy is to wait for many hours, with the final circulating pressure held on the casing, until the cement is hard. This is time consuming and costly and, in addition, can lead to technical problems with the well during its lifetime.
It is amongst the aims and objects of the invention to provide a method of testing float equipment (such as float shoes and float collars) which addresses one or more deficiencies of the conventional checks. It is one object of the invention to provide an apparatus and a method which provides for efficient and effective testing of float equipment after manufacture, but prior to use at the oil or gas well site. A further aim of the invention is to provide an apparatus and method for testing float equipment under conditions closer to the conditions experienced in use (and in particular, under the loads experienced during use). It is a further aim of the invention to provide an apparatus and method which is flexible enough to be used with a wide range of float equipment designs and sizes.
Further aims and objects of the invention will become apparent from the following description.